Surface acoustic wave device

ABSTRACT

A surface acoustic wave device includes an asymmetrical double electrode which prevents a mismatch between reflected waves and propagating surface acoustic waves on strips, and which is capable of realizing a superior unidirectionality. This surface acoustic wave device includes the asymmetrical double electrode in which a half wavelength section includes first and second strips which have mutually different widths. The half wavelength is arranged to define a basic section. The surface acoustic wave device includes at least two of these basic sections disposed on a piezoelectric substrate. The absolute value of the vector angle of the reflection center is within approximately 45±10° or within approximately 135±10°, when the center of the basic section is the reference position. Alternatively, the absolute value of the phase difference between the excitation center and the reflection center is within approximately 45±10° or approximately 135±10°.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a surface acoustic wave devicefor use in, for example, a resonator or a filter, and more particularly,to a surface acoustic wave device having an asymmetrical doubleelectrode used as a unidirectional interdigital transducer or adispersive reflection type reflector.

[0003] 2. Description of the Related Art

[0004] A surface acoustic wave device such as a surface acoustic wavefilter is widely used in mobile communication equipment or broadcastingequipment, or other such apparatuses. Particularly because the surfaceacoustic wave device is compact, lightweight, tuning-free and easy tomanufacture, the surface acoustic wave device is suitable for anelectronic component for use in portable communication equipment.

[0005] The surface acoustic wave device is broadly divided into atransversal type filter and a resonator-type filter, based on itsstructure. In general, the transversal type filter has advantages ofhaving (1) a small group delay deviation, (2) a superior phaselinearity, and (3) a high degree of flexibility in the pass band designbased on weighting. However, the transversal type filter has adisadvantage of having a large insertion loss.

[0006] An interdigital transducer (hereinafter referred to as an “IDT”)used in a surface acoustic wave filter transmits and receives surfaceacoustic waves with respect to both sides of an IDT, that is, the IDTtransmits and receives surface acoustic waves bilaterally in an equalmanner. For example, in a transversal type filter in which two IDTs arespaced apart from each other by a predetermined distance, one half ofthe surface acoustic waves transmitted from one IDT is received by theother IDT, but the surface acoustic waves propagated from the one IDT tothe opposite side of the other IDT become a loss. This loss is called a“two-way loss”, and has become a big factor in increasing insertion lossof a transversal type filter.

[0007] In order to reduce the above-described two-way loss, varioustypes of unidirectional IDTs have been proposed. In such unidirectionalIDTS, surface acoustic waves are transmitted and received at only oneside alone thereof. Also, low-loss transversal type filters whichutilize these unidirectional IDTs have been developed.

[0008] For example, Hanma et al., have proposed an asymmetrical doubleelectrode in “A TRIPLE TRANSIT SUPPRESSION TECHNIQUE” 1976 IEEEUltrasonics Symposium Proceedings pp. 328-331. FIG. 14 is a schematicpartially cutaway plan view showing the asymmetrical double electrodedisclosed in this prior art.

[0009] In an asymmetrical double electrode 101, half wavelength sectionsZ constituted of two strips 102 and 103 having different widths fromeach other, are disposed repeatedly many times along the propagationdirection of surface acoustic waves. Such an electrode defined by halfwavelength sections Z constituted of two strips having different widthsfrom each other, is called an “unbalanced double electrode” or a“asymmetrical double electrode”.

[0010] The width of a half wavelength section is set to 0.5λ. The widthof a strip 102 having a relatively narrow width is set to λ/16. Thewidth of a strip 103 having a relatively wide width is set to 3λ/16. Thewidth of a gap between the strips 102 and 103 is set to 2λ/16. The widthof an outer gap of the strip 102 in the half wavelength section is setto λ/16. The width of the outer gap of the strip 103 in the propagationdirection of surface acoustic waves in the half wavelength section isset to λ/16.

[0011] Between adjacent basic sections, the electrical polarities areopposite to each other.

[0012] In the above-described asymmetrical double electrode, areflection per basic section can be expressed by a resultant vector thatis generated by synthesizing reflected waves from the edges λ1 to λ4 ofthe strips 102 and 103 shown in FIG. 15. FIG. 16 shows the reflectionvectors at the edges λ1 to λ4 when the reference position is set to thecenter of a basic section, and the resultant vector thereof. As can beseen from FIG. 16, the resultant vector V is located at an angle of67.5°, and the reflection center is located at an angle of67.5°/2=33.75°.

[0013] Also, in this asymmetrical double electrode, the outer edge λ1 ofthe strip 102 and the outer edge λ4 of the strip 103 are disposedbilaterally symmetrically with respect to the center of the halfwavelength section. Hence, the distances between the center of a basicsection and the outer edges of the nearest strips in the adjacent basicsections, are also equal to each other. In the asymmetrical doubleelectrode, therefore, an excitation center is located at the center ofthe basic section Z, with a phase difference of about 33.75° generatedbetween an excitation center and the reflection center. Thus, theasymmetrical double electrode operates as a unidirectional electrode.

[0014] Table 1 below shows the inter-mode coupling coefficient κ₁₂/k₀,the phase difference between the excitation center ψ and the reflectioncenter φ, and the reflection center φ, when forming an asymmetricaldouble electrode of aluminum film having a 3% film-thickness on a ST-cutcrystal quartz substrate, as an example of the above-describedasymmetrical double electrode. TABLE 1 Item Calculated value Inter-modecoupling coefficient κ₁₂/k₀ 0.00257 Phase difference between excitationcenter Ψ 31.3° and reflection center φ Reflection center φ 33.8°

[0015] Here, k₀ is a wave number of surface acoustic waves propagatingthrough an IDT. The ratio κ₁₂/k₀ and the phase difference between theexcitation center ψ and the reflection center φ can be obtained from theresonant frequency determined by the finite element method, using thetechnique of Obuchi et al., (“Evaluation of Excitation Characteristicsof Surface Acoustic Wave Interdigital Electrode Based on Mode CouplingTheory”, Institute of Electronics, Information and CommunicationEngineers of Japan, Technical Report MW90-62). Also, the reflectioncenter φ is determined by the phase difference between the excitationcenter ψ and the reflection center φ, and the excitation center obtainedfrom the fundamental wave component which is acquired byFourier-transforming the electric charge density distribution on theelectrode obtained by the finite element method.

[0016] Japanese Unexamined Patent Application Publication No. 61-6917discloses an electrode which has implemented unidirectionality bydisposing two strips having mutually different widths in a halfwavelength section, as in the case of the above-described asymmetricaldouble electrode. The electrode disclosed in this Japanese UnexaminedPatent Application Publication No. 61-6917 is also supposed to operateas a unidirectional electrode due to the asymmetry of the two stripsthereof. However, in the method disclosed in the Japanese UnexaminedPatent Application Publication No. 61-6917, no means for controlling thereflection center and the reflection amount are disclosed. In addition,no feasible reflection center and reflection amount are described.

[0017] The article “Direct Numeral Analysis SAW Mode Coupling Equationand Applications Thereof”, 27th EM symposium preprint, pp. 109-116,Takeuchi et al., describes the principle of a unidirectional IDT whichprovides flat directivity over a wide band in the structure whereinpositive and negative reflection elements are dispersively disposed in aunidirectional IDT. Herein, however, no means for forming a reliablysuperior unidirectional IDT are described.

[0018] In general, when surface acoustic waves are caused to be incidenton an IDT constituted only of double strips without reflection,reflection is caused by re-excitation. As a result, in the case of aconventional transversal type filter, waves called “triple transit echo”or TTE, occur, and cause ripples or other undesired wave characteristicsthat adversely effect filter characteristics. The above-describedliterature by Hanma et al., discloses a method for canceling outreflection due to re-excitation by means of acoustic reflected waves ofan asymmetrical double electrode. This method, however, has created aproblem that new ripples are caused by acoustic reflection when theacoustic reflection is larger than the reflection caused by there-excitation. Therefore, such a method for canceling out the reflectionby re-excitation is subjected to the restriction of piezoelectricsubstrate material or electrode material, since the reflection vectorlength which represents the acoustic reflection amount is fixed in anasymmetrical double electrode.

[0019] On the other hand, the article “About One Weighting Method ForSAW Reflector”, 1999, General Convention of Institute of Electronics,Information and Communication Engineers of Japan, p. 279, Tajima et al.,discloses a method for performing weighting with respect to thereflection coefficient of a reflector. This method uses a plurality ofstrips having mutually different widths and makes use of the change ofthe reflection coefficient of a strip based on the strip width. However,when the strip width is changed, the sonic speed is also changed. As aresult, when attempting to perform weighting based on the strip width, atesting method and apparatus is needed to find a correct sonic speed andto change the arrangement pitch of the strip in accordance with thiscorrected sonic speed. This poses a problem that the design requires anextremely high degree of technique.

[0020] As described above, various IDTs or resonators each operating asa unidirectional electrode by asymmetry of two strips have beenproposed, but conventional asymmetrical double electrodes have not yetachieved sufficient unidirectionality. In addition, the reflectioncenter and the reflection amount of the conventional asymmetrical doubleelectrodes have been very difficult to control.

SUMMARY OF THE INVENTION

[0021] In order to overcome the problems described above, preferredembodiments of the present invention provide a surface acoustic wavedevice using an asymmetrical double electrode which achieves moresuperior unidirectionality of surface acoustic wave propagation whileeffectively and easily controlling the reflection amount per basicsection.

[0022] In accordance with a preferred embodiment of the presentinvention, a surface acoustic wave device includes a piezoelectricsubstrate, and including at least two basic sections including anasymmetrical double electrode in which a half wavelength sectionincludes first and second strips having different widths from eachother, the at least two basic sections being disposed along thepropagation direction of surface acoustic waves. In this surfaceacoustic wave device, the absolute value of the vector angle of thereflection center obtained from the resultant vector generated bysynthesizing the reflection vectors at the edges of the first and secondstrips is preferably within approximately 45±10° or approximately135±10°, when the center of the each of the at least two basic sectionsis the reference position.

[0023] In accordance with another preferred embodiment of the presentinvention, a surface acoustic wave device includes a piezoelectricsubstrate, and including at least two basic sections including anasymmetrical double electrode in which a half wavelength sectionincludes first and second strips having different widths from eachother, the at least two basic sections being disposed along thepropagation direction of surface acoustic waves. In this surfaceacoustic wave device, the absolute value of the phase difference betweenthe excitation center and the reflection center of the asymmetricaldouble electrode, is preferably within approximately 45±10° orapproximately 135±10°.

[0024] In accordance with a still another preferred embodiment of thepresent invention, a surface acoustic wave device includes apiezoelectric substrate, and including at least two basic sectionsincluding an asymmetrical double electrode in which a half wavelengthsection includes first and second strips having different widths fromeach other, the at least two basic sections being disposed along thepropagation direction of surface acoustic waves. In this surfaceacoustic wave device, when the edge positions of the first and secondstrips are X1 to X4, each of which is a value corrected using the sonicspeed difference between a free surface and a metallic surface, and whenthe resultant vector length of normalized reflected waves from the stripedges is |Γ|, and the center position of the basic section is 0(λ), andX1≅λ−4, each of the positions of X2 and X3 is a value substantiallysatisfying the following equations (1) and (2).

[0025] Mathematical Expression 4×

X2[λ]=A×X1[λ]² +B×X1[λ]+C±0.1[λ]  (1)

[0026] Mathematical Expression 5

X3[λ]=D×X1[λ]² +E×X1[λ]+F±0.05[λ]  (2)

[0027] Mathematical Expression 6

A=−34.546×|Γ|⁶+176.36×|Γ|⁵−354.19×|Γ|⁴+354.94×|Γ|³−160.44×|Γ|²+10.095×|Γ|−1.7558

B=−15.464×|Γ|⁶+77.741×|Γ|⁵−153.44×|Γ|⁴+147.20×|Γ|³−68.363×|Γ|²+6.3925×|Γ|−1.7498

C=−1.772×|Γ|⁶+8.7879×|Γ|⁵−17.07×|Γ|⁴+16.092×|Γ|³−7.4655×|Γ|²+0.8379×|Γ|−0.3318

D=12.064×|Γ|⁶−45.501×|Γ|⁵+57.344×|Γ|⁴−22.683×|Γ|³+12.933×|Γ|²−15.938×|Γ|−0.1815

E=7.2106×|Γ|⁶−30.023×|Γ|⁵+45.792×|Γ|⁴−29.784×|Γ|³+13.125×|Γ|²−6.3973×|Γ|+1.0203

F=1.0138×|Γ|⁶−4.4422×|Γ|⁵+7.3402×|Γ|⁴−5.474×|Γ|³+2.3366×|Γ|²−0.7540×|Γ|+0.2637

[0028] In the surface acoustic wave device in accordance with anotherpreferred embodiment of the present invention, it is preferable that thereflection amounts of the surface acoustic waves at the edge positionsX1 to X4 of the above-described strips be substantially equal to oneanother.

[0029] Also, in the surface acoustic wave device in accordance withother preferred embodiments of the present invention, theabove-described asymmetrical double electrode may be an interdigitaltransducer, or may instead be a reflector.

[0030] Furthermore, in accordance with another preferred embodiment ofthe present invention, preferably, quartz crystal is preferably used asthe above-described piezoelectric substrate. Alternatively, however, inother preferred embodiments of the present invention, the piezoelectricsubstrate may be constituted of another piezoelectric single crystalsuch as LiTaO₃, or a piezoelectric ceramic such as lead titanatezirconate-based ceramic. Also, a piezoelectric substrate constructed byforming a piezoelectric thin-film such as a ZnO thin-film on aninsulative substrate such as a piezoelectric substrate or aluminasubstrate, may be used.

[0031] The above and other elements, characteristics, features, andadvantages of the present invention will be clear from the followingdetailed description of preferred embodiments of the present inventionin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1A is a plan view of an asymmetrical double electrode inaccordance with a preferred embodiment of the present invention;

[0033]FIG. 1B is a partially cutaway sectional view of an asymmetricaldouble electrode in accordance with a preferred embodiment of thepresent invention;

[0034]FIG. 2 is a diagram showing the edge-position dependence of theexcitation center of the asymmetrical double electrode in a preferredembodiment of the present invention;

[0035]FIG. 3 is a diagram showing the relationship between the edgeposition X1=−X4 and each of the edge positions X2 and X3, when aresultant vector length Γ is 0.20λ.

[0036]FIG. 4 is a diagram showing the relationship between the edgeposition X1=−X4 and each of the edge positions X2 and X3, when aresultant vector length Γ is 0.50λ.

[0037]FIG. 5 is a diagram showing the relationship between the edgeposition X1=−X4 and each of the edge positions X2 and X3, when aresultant vector length Γ is 1.00λ.

[0038]FIG. 6 is a diagram showing the relationship between the edgeposition X1=−X4 and each of the edge positions X2 and X3, when aresultant vector length Γ is 1.25λ.

[0039]FIG. 7 is a diagram showing the relationship between the edgeposition X1=−X4 and each of the edge positions X2 and X3, when aresultant vector length Γ is 1.50λ.

[0040]FIG. 8 is a diagram showing the relationship between the edgeposition X1=−X4 and each of the edge positions X2 and X3, when aresultant vector length Γ is 1.70λ.

[0041]FIG. 9 is a diagram showing the change in the reflection center φwhen the edge position X2 obtained by the equation (1) changes, inpreferred embodiments of the present invention.

[0042]FIG. 10 is a diagram showing the change in the reflection center φwhen the edge position X3 changes in preferred embodiments of thepresent invention.

[0043]FIG. 11 is a schematic plan view showing the electrode structure,for evaluating directivity of an IDT in accordance with anotherpreferred embodiment of the present invention.

[0044]FIG. 12 is a diagram showing the relationship between the numberof basic sections and the directivity, which relationship has beenobtained in a further preferred embodiment of the present invention, andthe relationship between the number of the basic sections and thedirectivity when using a conventional asymmetrical double electrodeprepared for comparison.

[0045]FIG. 13 is an explanatory plan view of the electrode structure ofan IDT having a reflector in accordance with yet another preferredembodiment of the present invention.

[0046]FIG. 14 is a schematic partially cutaway plan view showing aconventional asymmetrical double electrode.

[0047]FIG. 15 is a partially cutaway sectional view for explaining theedge positions of the strips in the asymmetrical double electrode shownin FIG. 14.

[0048]FIG. 16 is a diagram showing the relationship between thereflection vectors in the edges X1 to X4 shown in FIG. 15 and theresultant vector V thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0049] In order to realize the unidirectionality using an asymmetricaldouble electrode, the inventors of the present application haveconducted extensive research and have discovered that, when thereflection amount of surface acoustic waves per basic section is small,the frequency unidirectionality characteristics of the unidirectionalelectrode can be estimated by forming reflection elements using aunidirectional electrode wherein the phase difference between theexcitation center and the reflection center is approximately +45°(−135°) or approximately −45° (+135°), and by disposing these positiveand negative reflection elements, regarding them as positive andnegative impulses, respectively. Furthermore, the present inventors havediscovered that, when the phase difference between the excitation centerand the reflection center largely deviates from approximately ±45°(±135°) in the positive and negative elements, it becomes difficult toregard as the positive and negative elements as simple positive andnegative impulses, respectively, because of the phase mismatching ofsurface acoustic waves.

[0050] Moreover, the present inventors have discovered that, when aweighting method in a unidirectional IDT using an asymmetrical doubleelectrode is used, it is possible to perform weighting with respect toreflection coefficients, when positive and negative reflection elementswherein the reflection centers thereof are located at angles ofapproximately ±45° (−135°) and −45° (+135°), respectively, with respectto the center of a half wavelength section, are formed and are utilizedas a reflector. When attempting to perform weighting to the strip width,it has been necessary to change the electrode pitch. However, thisweighting method by reflection coefficient allows a reflector to beeasily produced, since sonic speeds of the positive and negativeelements are identical with each other.

[0051] Next, the principles of various preferred embodiments of thepresent invention will be described in more detail with reference to thedrawings.

[0052] An asymmetrical double electrode 1 shown in FIGS. 1A and 1B istaken as an example. In this asymmetrical double electrode 1, basicsections Z each of which is constituted of strips 2 and 3 havingmutually different widths, are repeatedly arranged in the propagationdirection of surface acoustic waves. Now, let one basic section bedisposed at the positions from −0.25λ to +0.25λ. Here, λ denotes thewavelength of a surface acoustic wave.

[0053] Letting the positions of the edges of the first and second strips2 and 3 be disposed within this basic section, that is, this halfwavelength section be X1′ to X4′, and the sonic speed of surfaceacoustic waves propagating through a free surface be V_(f), and thesonic speed of surface acoustic waves propagating through a metallicsurface be V_(m), the edge positions X1 to X4 corrected based on thesonic speeds of the free surface and the metallic surface are expressedby the following equation:

[0054] Mathematical Expression 7

X1 to X4=(V _(f) L _(m) +V _(m) L _(f))/(V _(f) L _(m0) +V _(m) L_(f0))  (3)

[0055] In the above equation (3), L_(m) denotes the sum of the distanceon the metallic surface from the center of the half wavelength section,that is, 0λ to X1 to X4 in the propagation direction of surface acousticwaves, and L_(f) denotes the sum of the distance on the free surfacefrom the center of the half wavelength section, 0λ to X1 to X4. L_(m0)denotes the sum of the distance of the metallic surface in the entirehalf wavelength section, and L_(f0) denotes the sum of the distance ofthe free surface in the entire half wavelength section.

[0056] Next, the reflection in a single electrode in which only a singlestrip is disposed within the half wavelength section, will be discussed.Suppose that the single strip is arranged so that the center thereof islocated at the reference position 0λ of the half wavelength section Z.Letting the reflection vector at the one edge position −Xs of the singlestrip be Γs1, and the reflection vector at the other edge position +Xsthereof be Γs2, the resultant reflection vector Γs at the referenceposition is expressed by the equation (4) below. Here, j in the equation(4) denotes an imaginary number, and k denotes the wave number.

[0057] Mathematical Expression 8

Γs=Γs1×e ^(−2·j·k·(−Xs)) +Γs2×e ^(−2·j·k·Xs)  (4)

[0058] The length |Γs| of the above-described resultant vector Γsdenotes the reflection amount of a single strip.

[0059] Here, when conducting a normalization such as |Γs1|=|Γs2|=1, wecan express Γs1=−Γs2=−1 under the condition that the acoustic impedanceon a free surface is larger than that on a metallic surface. Therefore,when defining the reflection center φs as the center of the singlestrip, the reflection center φs can be determined by the followingequation (5), using the angle ∠Γ of the resultant reflection vector Γ.

[0060] Mathematical Expression 9

φs=−0.5×∠(j×Γs)  (5)

[0061] Next, discussion will be made of an asymmetrical double electrodewherein two strips having mutually different widths are disposed in thehalf wavelength section, as in the case of the single strip. Letting thereflection vectors of surface acoustic waves at the edge positions X1 toX4 in FIGS. 1A and 1B be Γ1 to Γ4, the resultant reflection vector Γ atthe reference position 0λ is expressed by the equation (6) below.

[0062] Mathematical Expression 10

Γ=Γ1×e ^(−2·j·k·X1)+Γ2×e ^(−2·j·k·X2)+Γ3×e ^(−2·j·k·X3)+Γ4×e^(−2·j·k·X4)  (6)

[0063] The length |Γ| of the above-described resultant vector Γ denotesthe reflection amount of a unidirectional electrode. The reflectioncenter of the unidirectional electrode is defined in the same way as thesingle strip, and is expressed by the equation (7) below.

[0064] Mathematical Expression 11

φ=−0.5×∠(j×Γ)  (7)

[0065] In the case where, in the asymmetrical double electrode, aunidirectional IDT is constructed such that the electric polarities ofadjacent basic sections are alternately inverted, when the width of theinter-strip gap between a basic section and the adjacent basic sectionon one side in the propagation direction of surface acoustic waves, andthe width of the inter-strip gap between the basic section and theadjacent basic section on the other side in the propagation direction ofsurface acoustic waves, are equal to each other, and simultaneously whenthese inter-strip gaps are disposed symmetrically with respect to thecenter of the center basic section, the excitation center of theasymmetrical double electrode is located at the substantially centralportion of the half wavelength section.

[0066]FIG. 2 is a diagram showing the edge-position dependence of theexcitation center in the above-described asymmetrical double electrode.Herein, an asymmetrical double electrode formed of an aluminum filmhaving a thickness of, for example, approximately 0.02λ, is disposed ona ST-cut quartz substrate. In this figure, there is shown the edgeposition dependence of the excitation center obtained from thefundamental wave component which is acquired by Fourier-transforming theelectric charge density distribution on the electrode obtained by thefinite element method, when X1=−X4=−0.1875λ, and X3−X2=0.125λ, and whenX2 is used as a parameter.

[0067] It can be confirmed that even at a position wherein the degree ofasymmetry of the asymmetrical double electrode is very high, that is, atX2=0.172λ, the vector angle of the excitation center is located at about+4.6°, that is, substantially at the central portion. The strip widthand the gap width of an IDT constituting a surface acoustic wave deviceis restricted by the electrical resistance of a strip and/or thepatterning process.

[0068] The edge positions X and X3 can be uniquely determined withrespect to the |Γ| and the edge position X1, by letting X2−X1>0.02λ,X3−X2>0.02λ, X4−X3>0.02λ, and X4=−1, assuming that the vector lengths ofΓ1 to Γ4 are equal to one another, performing a normalization such thatΓ1=Γ4=−1, Γ2=Γ3=+1, and finding the conditions such that the equations(6) and (7) satisfies φ=45°, by the Monte Carlo method. The approximateequations expressing X2 and X3 are given by the following expressions(8) and (9), using |Γ| and X1 as independent variables.

[0069] Mathematical Expression 12

X2[λ]≅A×X1[λ]² +B×X1[λ]+C  (8)

[0070] Mathematical Expression 13

X3[λ]≅D×X1[λ]² +E×X1[λ]+F  (9)

[0071] In the equations (8) and (9), A to F are obtained by thefollowing equations.

[0072] Mathematical Expression 14

A=−34.546×|Γ|⁶+176.36×|Γ|⁵−354.19×|Γ|⁴+354.94×|Γ|³−160.44×|Γ|²+10.095×|Γ|−1.7558

B=−15.464×|Γ|⁶+77.741×|Γ|⁵−153.44×|Γ|⁴+147.20×|Γ|³−68.363×|Γ|²+6.3925×|Γ|−1.7498

C=−1.772×|Γ|⁶+8.7879×|Γ|⁵−17.07×|Γ|⁴+16.092×|Γ|³−7.4655×|Γ|²+0.8379×|Γ|−0.3318

D=12.064×|Γ|⁶−45.501×|Γ|⁵+57.344×|Γ|⁴−22.683×|Γ|³+12.933×|Γ|²−15.938×|Γ|−0.1815

E=7.2106×|Γ|⁶−30.023×|Γ|⁵+45.792×|Γ|⁴−29.784×|Γ|³+13.125×|Γ|²−6.3973×|Γ|+1.0203

F=1.0138×|Γ|⁶−4.4422×|Γ|⁵+7.3402×|Γ|⁴−5.474×|Γ|³+2.3366×|Γ|²−0.7540×|Γ|+0.2637

[0073] From the above results, it can be recognized that an asymmetricaldouble electrode which corresponds to a desired reflection amount, andhaving the reflection center at an angle of about 45° can be obtained.As can further be recognized, in an asymmetrical double electrode whichis constructed in accordance with the equations described above, theexcitation center is located at the center of a half wavelength section.As a result, when this asymmetrical double electrode is used as aunidirectional electrode, the phase difference between the excitationand the reflection center becomes substantially 45°, allowing thisasymmetrical double electrode to operate as a unidirectional electrodehaving very superior characteristics.

[0074] As examples, FIGS. 3 to 8 show the results of X2 and X3 obtainedby equations (8) and (9), for |Γ|=0.20λ, 0.50λ, 1.00λ, 1.25λ, 1.50λ, and1.70λ. Meanwhile, in the above description, the reflection coefficienthas been treated based on the premise that the acoustic impedance on afree surface is larger that that on a metallic surface. Conversely,under the condition that the acoustic impedance on a free surface issmaller that that on a metallic surface, only the sign of |Γ| isreversed, or in other words, that φis shifted by 90°.

[0075] As described above, by selecting the edge positions X2 and X3 soas to satisfy the equations (8) and (9), the phase difference betweenthe excitation center and the reflection center can be madesubstantially 45°. As a result, a very superior unidirectional electrodecan be achieved. However, the present inventors have confirmed that thisasymmetrical double electrode has a very excellent unidirectionality, ifX2 and X3 are located not only at the positions satisfying the equations(8) and (9), but also at the positions within a certain range from thepositions satisfying the equations (8) and (9). This fact will bedescribed with reference to FIGS. 9 and 10.

[0076]FIGS. 9 and 10 are diagrams each showing the changes in thereflection center when X2 and X3, each obtained by substituting |Γ|=1.5and X1=−0.2188λ into the equations (8) and (9), within the range from−0.1λ to +0.1λ.

[0077] As described above, it is desirable that the reflection center belocated at an angle of approximately 45°, or the phase differencebetween the reflection center and the excitation center be approximately45°, but the present inventors have confirmed that the range withinapproximately 45+10° would allow the phase mismatching to be greatlyimproved as compared to the above-described prior art asymmetricaldouble electrode. It can be seen from FIGS. 9 and 10 that the range suchthat the position of the reflection center is at an angle ofapproximately 45+10°, corresponds to the range of about ±0.10λ withrespect to the value obtained by the equation (8) for the position ofX2, and corresponds to the range of about ±0.05λ with respect to thevalue obtained by the equation (9) for the position of X3.

[0078] In preferred embodiments of the present invention, therefore, thepositions of X2 and X3 are preferably within the range shown in theabove-described equations (1) and (2). It will be understood that asuperior unidirectionality can be realized as a result of this uniquearrangement.

[0079] A surface acoustic wave device using an asymmetrical doubleelectrode in accordance with preferred embodiments of the presentinvention was constructed as illustrated in FIG. 1. An IDT wasconstructed by forming an aluminum film having a thickness of, forexample, approximately 0.02λ on a ST-cut quartz substrate, and thenperforming patterning.

[0080] The IDT defining an asymmetrical double electrode was constructedin accordance with the edge positions X2 and X3 which were determined bysubstituting the values of |Γ| and X1 shown in Table 2 below into theequations (8) and (9). Table 2 shows the inter-mode couplingcoefficients κ₁₂/k₀ and the reflection centers φ in this case.

[0081] In the asymmetrical double electrode, shown in FIG. 2, which isconstructed based on the equations (8) and (9), since the angle of thereflection center is close to 45°, the phase mismatching of thereflected waves with respect to the propagating waves is significantlyless than that of the conventional asymmetrical double electrode.Therefore, the use of the asymmetrical double electrode constructedbased on the equations (8) and (9), allows a surface acoustic wavedevice which performs much better than the conventional surface acousticwave devices to be achieved, and is particularly effective whenpositively making use of the reflection of strips. TABLE 2 Reflection|Γ| X1 [λ] κ₁₂/k₀ center φ [λ] 0.20 −0.19 0.0005 43.8 0.50 −0.20 0.001542.3 0.75 −0.20 0.0021 40.5 1.00 −0.21 0.0029 39.6 1.25 −0.21 0.003139.0 1.50 −0.22 0.0036 39.2 1.60 −0.22 0.0035 40.2 1.70 −0.23 0.003841.5 1.73 −0.23 0.0038 42.2

[0082] Next, description will be made of specific experimental examplesof the directivity when an IDT including an asymmetrical doubleelectrode is provided on a ST-cut quartz substrate, in accordance with apreferred embodiment of the present invention.

[0083] As shown in FIG. 11, IDT 11, IDT 12, and IDT 13 were formed on aST-cut quartz substrate (not shown) using an aluminum film having athickness of, for example, approximately 0.02λ. The middle IDT 11 isconstituted of an asymmetrical double electrode in accordance withpreferred embodiments of the present invention, and IDT 12 and IDT 13disposed on the opposite sides of IDT 11 are ordinary double electrodetype IDTs.

[0084] In IDT 11 constituted of an asymmetrical double electrode, whenthe edge portions of the first and second strips 2 and 3 havingdifferent widths are made asymmetric, the excitation center deviatesfrom the center of the half wavelength section, so that the phasedifference between the excitation and the reflection center alsodeviates from approximately 45°. Therefore, the edge positions X2 and X3obtained by substituting |Γ|=1.5, and X1=−0.2188λ into the equations (8)and (9), were adjusted by about 0.05λ and corrected so that the phasedifference between the excitation and the reflection center approachesapproximately 45°.

[0085] As a result, when X1=−0.2188λ, X2=−0.1185λ, X3=+0.0050λ, andX4=+0.2188λ, the phase difference between the excitation center and thereflection center became about 41°.

[0086]FIG. 12 shows the comparison between the directivity of IDT 11which uses the electrode structure shown in FIG. 11 and which includesthe asymmetrical double electrode having the above-describedconstruction, and the directivity when the conventional asymmetricaldouble electrode is disposed in place of IDT 11. The solid line in thefigure shows the result of IDT 11, and the broken line shows that of theconventional example. With regard to the directivity, an input voltageis applied to IDT 11, then the output thereof received by IDT 12 and IDT13 was sought, and the directivity was evaluated from the value of thisoutput (dB).

[0087] For an IDT using an asymmetrical double electrode prepared forcomparison, the film thickness of the electrode was preferably set toabout 0.02λ, and the edge positions were preferably set so as to beX1=−0.1875λ, X2=−0.1250λ, X3=0λ, and X4=+0.1875λ. The crossing width ofan electrode finger was preferably set to about 20λ in each of thepreferred embodiments and the conventional example.

[0088] For IDT 12 and IDT 13 on the opposite sides of IDT 11, thecrossing width of an electrode finger were preferably set to about 20λ,and the edge positions were preferably set so as to be X1=−0.1875λ,X2=−0.0625λ, X3=+0.0625λ, and X4=+0.1875λ.

[0089] It can be recognized from FIG. 12 that the asymmetrical doubleelectrode in this preferred embodiment has a better unidirectionalitythan that of the conventional asymmetrical double electrode. Inaddition, the present inventors have confirmed that the phase differencebetween the excitation center and the reflection center can be correctedso as to approach 45° by adjusting the edge positions X2 and X3 obtainedby the equations (8) and (9) by about ±0.1λk, or by adjusting X4 so asto slightly depart from −X1.

[0090]FIG. 13 is a plan view showing the electrode structure of an IDThaving an reflector 21 according to yet another preferred embodiment ofthe present invention. Herein, the reflector 21 constructed inaccordance with this preferred embodiment of the present invention ispreferably disposed within IDT 22. In this case, by performing weightingwith respect to the reflection coefficient of the reflector 21, it ispossible to control the frequency characteristics of the entire IDT 22having the reflector 21.

[0091] The present invention is not limited to the above-describedpreferred embodiments, but can be variously modified. For example, inthe above-described preferred embodiments, it is recognized that abetter directivity than that of the conventional example is achieved.However, there may be a case, depending on the use, where it is moreimportant that the phase difference between the excitation center andthe reflection center is close to 45°, or that the reflection centerwhen X1=−X4, is 45° with respect to the center of the half wavelengthsection, rather than achieving better directivity. Although it isdesirable that the phase difference between the excitation center andthe reflection center be about 45°, there may be a case where, when thereflection by a strip is positively utilized, for example, when it isused as a reflector, priority is given to the feature that thereflection center is located at an angle of 45°, over the feature thatthe excitation center is located at the center of the half wavelengthsection, even if the excitation center deviates therefrom. Particularlywhen a strip is utilized as a reflector, only the reflection center canbe taken into consideration.

[0092] As is evident from the foregoing, in the surface acoustic wavedevice using an asymmetrical double electrode in accordance with variouspreferred embodiments of the present invention, the absolute value ofthe vector angle of the reflection center obtained from the resultantvector formed by synthesizing the reflection vectors at the edges X1 toX4 of the first and second strips when the center of the above-describedbasic section is set to be the reference position, is preferably withinapproximately 45±10° or approximately 135±10°. Thereby, the phasemismatching of surface acoustic waves is minimized, and theunidirectionality of the above-described asymmetrical double electrodeis greatly improved.

[0093] Likewise, in various preferred embodiments of the presentinvention, when the absolute value of the phase difference between theexcitation center and the reflection center of the asymmetrical doubleelectrode, is within approximately 45±10° or approximately 135±10°, thephase mismatching of surface acoustic waves is minimized, and superiorunidirectionality can be realized.

[0094] In preferred embodiments of the present invention, in the edgepositions X1 to X4 of the first and second strips, which constitutebasic sections and which have mutually different widths, when the centerposition of the basic section is 0(λ), and X1≅−X4, if the positions ofX2 and X3 satisfy the equations (1) and (2), it is ensured that theabsolute value of the vector angle of the reflection center is withinapproximately 45±10° or approximately 135±10° when the center of thebasic section is the reference position, or that the absolute value ofthe phase difference between the excitation center and the reflectioncenter is within approximately 45±10° or approximately 135±10°. It is,therefore, possible to easily and reliably provide, in accordance withpreferred embodiments of the present invention, an asymmetrical doubleelectrode which prevents the phase mismatching of surface acousticwaves, and which has a superior unidirectionality.

[0095] When the reflection amounts of surface acoustic waves at the edgepositions X1 to X4 are substantially equal to one another, the phasemismatching between reflected surface acoustic waves and propagatingsurface acoustic waves is very effectively reduced.

[0096] When an IDT is constructed to include the asymmetrical doubleelectrode, in accordance with various preferred embodiments of thepresent invention, the phase mismatching between reflected surfaceacoustic waves and propagating surface acoustic waves is prevented,thereby allowing an IDT having a superior unidirectionality to beprovided, and enabling, for example, a low-loss transversal type surfaceacoustic wave device to be provided.

[0097] When the asymmetrical double electrode in accordance withpreferred embodiments of the present invention is used as a reflector,since weighting can be easily performed with respect to the reflectioncoefficient, it is possible to provide a surface acoustic wave devicewhich is capable of controlling the frequency characteristics of theoverall reflection coefficient of reflectors.

[0098] While the present invention has been described with reference towhat are at present considered to be preferred embodiments, it is to beunderstood that various changes and modifications may be made theretowithout departing from the invention in its broader aspects andtherefore, it is intended that the appended claims cover all suchchanges and modifications as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A surface acoustic wave device, comprising: apiezoelectric substrate; and at least two basic sections disposed onsaid piezoelectric substrate, each of the at least two basic sectionsincluding an asymmetrical double electrode defining a half wavelengthsection and having first and second strips with different widths fromeach other; wherein an absolute value of a vector angle of a reflectioncenter obtained from a resultant vector generated by synthesizingreflection vectors at edges of the first and second strips, is within arange of angles of approximately 45±10° or approximately 135±10°, when acenter of a respective one of said at least two basic sections is areference position for the range of angles.
 2. A surface acoustic wavedevice according to claim 1, wherein reflection amounts of surfaceacoustic waves at edge positions of said strips are substantially equalto one another.
 3. A surface acoustic wave device according to claim 1,wherein said asymmetrical double electrode is an interdigitaltransducer.
 4. A surface acoustic wave device according to claim 1,wherein said asymmetrical double electrode is a reflector.
 5. A surfaceacoustic wave device according to claim 1, wherein said piezoelectricsubstrate is made of a quartz crystal material.
 6. A surface acousticwave device, comprising: a piezoelectric substrate; and at least twobasic sections disposed on said piezoelectric substrate, each of the atleast two basic sections including an asymmetrical double electrodedefining a half wavelength section and having first and second stripswith different widths from each other; wherein an absolute value of aphase difference between an excitation center and an reflection centerof said asymmetrical double electrode is within approximately 45±10° orapproximately 135±10°.
 7. A surface acoustic wave device according toclaim 6, wherein reflection amounts of surface acoustic waves at edgepositions of said strips are substantially equal to one another.
 8. Asurface acoustic wave device according to claim 6, wherein saidasymmetrical double electrode is an interdigital transducer.
 9. Asurface acoustic wave device according to claim 6, wherein saidasymmetrical double electrode is a reflector.
 10. A surface acousticwave device according to claim 6, wherein said piezoelectric substrateis made of a quartz crystal material.
 11. A surface acoustic wavedevice, comprising: a piezoelectric substrate; and at least two basicsections disposed on said piezoelectric substrate, each of the at leasttwo basic sections including an asymmetrical double electrode defining ahalf wavelength section and having first and second strips withdifferent widths from each other; wherein when edge positions of saidfirst and second strips are X1, X2, X3 and X4, each of which is a valuecorrected using a sonic speed difference between a free surface and ametallic surface, and when a resultant vector length of normalizedreflected waves from the edge positions is |Γ|, and a center position ofone of said at least two basic sections is 0(λ), and X1≅−X4, each of thepositions of X2 and X3 is substantially equal to a value satisfying thefollowing equations (1) and (2): X2[λ]=A×X1[λ]² +B×X1[λ]+C±0.1[λ]  (1)X3[λ]=D×X1[λ]² +E×X1[λ]+F±0.05[λ]  (2); wherein in the equation (1) and(2), A to F are expressed by the following equations:A=−34.546×|Γ|⁶+176.36×|Γ|⁵−354.19×|Γ|⁴+354.94×|Γ|³−160.44×|Γ|²+10.095×|Γ|−1.7558B=−15.464×|Γ|⁶+77.741×|Γ|⁵−153.44×|Γ|⁴+147.20×|Γ|³−68.363×|Γ|²+6.3925×|Γ|−1.7498C=−1.772×|Γ|⁶+8.7879×|Γ|⁵−17.07×|Γ|⁴+16.092×|Γ|³−7.4655×|Γ|²+0.8379×|Γ|−0.3318D=12.064×|Γ|⁶−45.501×|Γ|⁵+57.344×|Γ|⁴−22.683×|Γ|³+12.933×|Γ|²−15.938×|Γ|−0.1815E=7.2106×|Γ|⁶−30.023×|Γ|⁵+45.792×|Γ|⁴−29.784×|Γ|³+13.125×|Γ|²−6.3973×|Γ|+1.0203F=1.0138×|Γ|⁶−4.4422×|Γ|⁵+7.3402×|Γ|⁴−5.474×|Γ|³+2.3366×|Γ|²−0.7540×|Γ|+0.2637.12. A surface acoustic wave device according to claim 11, wherein thereflection amounts of surface acoustic waves at the edge positions X1,X2, X3 and X4 of said strips are substantially equal to one another. 13.A surface acoustic wave device according to claim 11, wherein saidasymmetrical double electrode is an interdigital transducer.
 14. Asurface acoustic wave device according to claim 11, wherein saidasymmetrical double electrode is a reflector.
 15. A surface acousticwave device according to claim 11, wherein said piezoelectric substrateis made of a quartz crystal material.